I'm an old timer TOS fan, way before "Minshara" came into the picture. Alls I ever knew was Class M, no explanation given. But way back when I was a kid...

Why M? Only thing I could think of was M being smack in the middle of the alphabet. Class M were the planets just right for human life.

Like some kind of Goldilocks thing, I thought the 26 letter alphabet was the gamut of planets. Class A planets were way too cold, Class Z planets were way too hot, but Class M planets were just right (smack in the middle).

Arbitrary? Probably. But at least I didn't invent some Minshara planets to explain the "M".

There haven't been that many in the episodes. I can recall:
Class D planetoids - rocky, no atmosphere, minor planets (presumably like Pluto)
Class K planets breathable but uninhabitable due to temperature extremes
Class ? What was the Demon planet again? Was it Y?
Class G - I might have dreamed this one but I think it was a gas giant.

I think some non-canon sources have tried to map out the classifications. Has one ever been formally adopted?

Yeah. I like to think that "Class-M" and "Minshara-class" just happen to both start with the letter M.

I'm not sure that temperature is the main qualifier of what class of planet one is, but is one of numerous factors in the over all habitability of a given world. Things like Atmosphere composisiton, gravity, exposure to radiation and other features no doubt play a role as well. But I agree that the closer to "M" in the middle are the habitable worlds.

Demon was considered class Y. Class K, such as the planet of Mudd's androids, could be inhabited but required structures like pressure domes and artificial habitats. Mars would fall under this category in the Star Trek: Star Charts.

Other canonical mentions include class T (includes some gas giants, mentioned in VOY "Good Shepherd"); Class J planets are also gas giants, similar to Saturn and Jupiter. (DS9 "Starship Down"). Class H is for areas generally uninhabitable by human life, although they could be adapted like Class K since the colony under Sheliak threat was on a class H world. Class H environments were primarily desert, whereas class K might have an intolerable atmosphere. Class N, mentioned briefly in "Night Terrors" but only elaborated on in Star Charts, is a greenhouse environment similar to Venus. Lots of gases and no liquid water. Class L planets, seen in DS9 and VOY, were "marginally" habitable planets with fewer natural resources than a class M, but where survival for a time was not impossible.

Star Charts also mentions a couple of other categories, including class F (geologically active with no atmosphere) and class P (glaciated with lots of surface ice, such as Andor and Psi 2000). The full list is on Memory Alpha.

And, there are types of asteroids from A to N. The type is based on size. ("Metamorphosis")

And, there are types of atmospheres from H to M. ("Metamorphosis")

I can't make sense of this.

Note: In the Star Trek: Star Charts, planetary classifications are determined by many factors, including the diameter of the planet. Interestingly, there is a gap - between 15,000 and 50,000 diameters - for which there is no classification, and Neptune falls into this range.

...Yeah, it took some time to realize that the letter wasn't being translated for our benefit in the subtitles, and was indeed M (rather than E) in the original dialogue as well.

We have heard various attributes (atmosphere composition, surface temperature) associated with certain classification letters, but we have also witnessed contradictions. Class D is appropriate both for the lifeless, airless rock Regula of ST2:TwoK, the large, solid body Weytahn from ENT "Cease Fire", and the ringed, atmosphered planet of VOY "Emanations".

So I'd argue that any individual parameter, such as atmosphere composition or temperature, is irrelevant to the classification. What counts is the end result, the sum total of attributes: M is something you can colonize simply by pitching a tent, K is something that requires airtight domes, and L requires something in between.

D calls for extensive use of technology if one wants to remain there even temporarily - but the moving down the alphabet from M towards A may also denote the decreasing usefulness of the world to the settlers. That is, D, E and F are all impossible to live on unless you live in a bubble and wear a spacesuit when you go out, but F can at least be mined economically, E could be mined if one went to a lot of effort, and there's nothing to mine on D.

Why the scale would also have to go "up" from M, I don't know. Clearly, the higher letters of the alphabet again denote planets that cannot be easily inhabited, but it sounds redundant to have both A and Z to denote a planet maximally difficult to colonize. Perhaps A stands for planets that are impossible to live on but also worthless, whereas Z is for planets that are impossible to live on but full of treasures? That would allow for Y in "Demon", as it did have enriched deuterium for immediately useful starship fuel, and for gas giants J and T both (because the gases involved might be worthless or valuable).

^"Not everything has to tie in?" An odd comment, given that the whole point of ENT was to show the origins of the Trek universe we knew. Naturally the intention was that "Minshara-class" would be the designation that eventually got shortened to M-class. I liked it. I liked the idea that something in standard Federation usage originated with the Vulcans instead of everything being Earth-based. ENT did a very good job of breaking free of that "everything is human in origin" assumption and showing that some familiar things originated with other species -- for instance, Vulcans and Andorians having deflector shields and tractor beams before Earth did, Klingons and Vissians having photon(ic) torpedoes first, Xyrillians inventing the holodeck, etc.

And no, we do not know canonically that these classes are just parts of an alphabet scale. That's a fan assumption, but we don't have enough letters canonically specified to know that for sure. Assumptions are not inviolable truths. And of course, even if there is an alphabet scale in the 23rd or 24th century, they could easily have chosen the letter M to represent Earth/Vulcan-like planets because of the established "Minshara-class" usage. That would make perfect sense and it's a bizarre thing to get angry about.

I bet plenty of scientists would work on the classifications for free!

That gives us a fair range of possibilities with enough scope to cover our list. They need to get to grips with a logical progression though. Class M sits in the middle so what should be either side of it? It would make more sense to me if they started with rocky, then gas, then ice with variations on the scale.

So Class A would be very hot, rocky, low atmosphere, low magnetic field, tidally locked, lifeless, close to a star - like Mercury.
Class B would be gas giant, hot, low atmospheric pressure, low magnetic field, tidally locked, lifeless, close to a star.
Class C would be gas giant, hot, high atmospheric pressure, low magnetic field, tidally locked, lifeless, close to a star and so on.

I suppose they could also adjust classification depending on whether a ship could safely pass through the atmosphere.

Back in 2006, I posted a thread in this forum that critiqued Star Charts's version of the planetary classification scheme and offered an alternative suggestion based on modern understanding of planetary sciences. It doesn't seem to be on the forum anymore, but fortunately I saved a copy, so I'll repost it here:

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Rethinking STAR CHARTS planet classification?

The other day, I was looking through the planetary classification scheme covered on pp. 22-29 of Star Charts (which is more or less a mix of the conjectural schemes from tie-in publications over the decades and the few letter classifications that have been established canonically), and I came to realize it has some flaws in its assumptions, particularly where gas giant planets are concerned.

I realized this when I discovered it has no classification specifically for Neptune-class planets. Neptune and Uranus are a distinctly different class of planet from Jupiter and Saturn, sometimes called "ice giants," because they have a different internal composition and structure including a greater proportion of water, ammonia and methane in ice and/or liquid form. But there's no class in Star Charts for that kind of planet. Indeed, Class J, the class including Jupiter and Saturn, is the smallest of the four proposed gas-giant types in SC. Class I is called a "Gas Supergiant" and is said to be considerably larger than class J, ranging up to 10 million kilometers, while classes S and T are said to be even more immense, ranging from 10-120 million km -- which doesn't even make sense, since the Sun is only 1.4 million kilometers across!

Also, when planets get as big as Jupiter or bigger, their cores are so compressed that they collapse into degenerate matter (the stuff white dwarfs are made of). So the more mass you pile onto them, the more you compress their cores, and that shrinking cancels out any size increase you'd get from adding more stuff. That means there aren't any gas giants or even brown dwarfs that are significantly bigger in diameter than Jupiter. 140-150,000 km is the biggest that just about any Jovian or brown dwarf is ever going to get. The only exception is if a Jovian is really close to its star, which would heat its atmosphere and cause it to expand.

This brings up the remaining flaw in SC's categorization of gas giants -- it assumes they would all be in the so called "Cold Zone" beyond the ecosphere. But in recent years we've discovered many "hot Jupiters," Jovians orbiting close to their stars, in other star systems, so we now know that Jovians can be found anywhere in a star system, not just in the outer reaches.

And a fellow named David Sudarsky has come up with a classification scheme for extrasolar Jovians as a function of their distance from their primary stars and thus their atmospheric temperatures, which would affect their appearance and composition.

So instead of the four Jovian classes in Star Charts, three of which are physically impossible, we probably need something like six -- one for each of the Sudarsky classes, plus one for Neptune-class ice giants. Since classes U, V, and W are undefined in the book, those could perhaps be used. Unfortunately, we're canonically stuck with Classes H and K being terrestrial planets and Class J being Jovian, so we can't put those six classes together.

Here are some other Star Charts classifications I have thoughts about:

Class A: Geothermal: A young, partially molten rocky planet. Cools to become Class C: Geoinactive, a dead, frozen planet with Pluto and Psi 2000 listed as examples. Now, the problem here is that Pluto is now classed as a dwarf planet, and that bodies like it are composed as much of ice as of rock.

Class B: Geomorteus (huh?): A hot rocky world close to a star, like Mercury -- except said to be "partially molten" on the surface, which Mercury isn't.

Class D: Asteroid/Moon. The problem here is that there should be more than one class of this. There are several distinct types of asteroids, including silicate, carbonaceous, icy/carbonaceous, or a mix. Luna is a mostly rocky moon, while most large moons in the outer system are a mix of rock and ice (indeed, Saturn's moon Mimas is almost entirely made of water ice). Some moons are bigger than some planets, while others would qualify as dwarf planets if they weren't satellites.

Class E through G: all intermediate stages in the cooling process of young worlds that become class K through P. Do we really need three of them?

Class O: Pelagic: a world with water over more than 80% of surface area. This is a reasonable category to have; recent findings suggest that the inward migration of "hot Jupiters" or the presence of a red dwarf companion could send tons of comets into the inner system to bombard the planets, possibly resulting in terrestrial planets with as much as 100 times as much water as Earth. So worlds that are mostly or entirely covered in water are quite possible. Although I'd set the dividing line between M and O somewhat higher, maybe 90%.

Class Q: Variable: Allegedly something that changes "due to eccentric orbit or variable output of star," but its example of the Genesis Planet is pretty iffy and the category is vague. It might make more sense to say that a planet changes from, say, Class L to class P conditions as it moves in its orbit.

Class R: Rogue, a planet expelled from its system like Dakala in ENT: "Rogue Planet." Actually more scientifically credible than it seems. This class might also encompass what are being called "planemos," planetary mass objects that formed independently in deep space. The problem is that the class doesn't distinguish between terrestrial and Jovian rogues.

Now, canonically, only ten letter classifications have been used: Classes D, H, J, K, L, M, N, P, T, and Y. That leaves the rest up for re-evaluation. And some of the canonical ones could have their details tweaked too without violating onscreen material.

So here's my tentative proposal for a revised classification scheme:

A: A young, molten planet or dwarf planet. (Gothos, Excalbia)

B: A moon or dwarf planet with an outer layer of water ice and perhaps a liquid-water mantle, found in the middle portions of a system. (Ceres, Europa)

C: A moon or dwarf with an outer layer of water, methane, ammonia and/or carbon dioxide ice, found in the outer part of a system. (Pluto, Triton)

D: A dry, silicate moon or dwarf planet. (Luna, Regula I)

E, F, G: Maybe make these lifeless rocky planets of varying sizes, from Mercury-sized to Earth-sized to super-Earth-sized.

H: Canonically, the H-class world Tau Cygna V is habitable but subjected to levels of hyperonic radiation that humans can't survive. Perhaps generally this could be a world that's habitable but too irradiated for humanoids.

I: A Neptune-class ice giant.

J: A Jupiter-class (Sudarsky I) gas giant.

K: Canonically, "adaptable with pressure domes." Say, something reasonably close to Earth gravity but lacking a dense or breathable atmosphere.

L: An almost Earthlike world, but with relatively little oxygen in the atmosphere. Essentially what Earth would've been about 400-500 million years ago.

M: Duh.

N: Let's stick with Mandel's idea of making this a Venus-like world, superdense atmosphere. Fits with the earlier classes being distinguished by their thin or oxygen-poor atmospheres.

O: Pelagic, again sticking with Mandel. Actually I'd like to have N and O the other way around for a better progression, but N is already sort of canonical.

It seems to be a scientifically based classification system. I think Christopher is on to something and if we look at adding in sub-categories to include common sci fi issues such as survivability using pressure suits and pressure domes and ship survivability as key points in the subcategories.

Planet Type Mass (Earth Units) Radius (Earth Units)Potential Atmosphere*
Asteroidan 0 - 0.00001 0 - 0.03Asteroidans are small irregular bodies (below the hydrostatic equilibrium) that are not able to hold a stable atmosphere.
Mercurian 0.00001 - 0.1 0.03 - 0.7Mercurians are only able to hold a significant atmospheres in the cold zones beyond the snow line (i.e. Titan).
Subterran 0.1 - 0.5 0.5 - 1.2Subterrans are able to hold a significant atmospheres after the outer edges of the habitable zone (i.e. Mars).
Terran 0.5 - 20.8 - 1.9
Terrans are able to hold a significant atmosphere with liquid water within the habitable zone (i.e. Earth).
Superterran 2 - 10 1.3 - 3.3Superterrans are able to hold dense atmospheres with liquid water within the habitable zone.
Neptunian 10 - 50 2.1 - 5.7Neptunians can have dense atmospheres in the hot zone.
Jovian 50 - 5000 3.5 - 27Jovians can have superdense atmospheres in the hot zone.
* The cold zone is the zone farther from the star after the habitable zone (which includes the snow line), the hot zone is the zone between the star and the habitable zone.

I'd argue that by the time of Star Trek, sufficiently many distinct planet types (that is, permutations on the dozens of relevant parameters) would have been discovered to make a letter-based system impractical - there'd be at least a hundred distinct permutations there, and probably a thousand, if we allow something like the mass difference between 0.5 and 1.5 Earths to be a distinction.

It's not as if our heroes would have much use for such categorizing of planets, either. It doesn't appear to reveal anything noteworthy about the geohistory or -future of the worlds, about the odds of life evolving there, of dilithium being there. It's just diversity forced into a format.

But presumably outside that mass range, gravity would be too great or too low to be class M anyway, proceeding on the assumption that a smaller planet's magnetic field would be too weak within the habitable zone to sustain its atmosphere long enough for life to develop. Although having said that, I think Venus' magnetic field is not all that strong presumably its atmosphere is so thick because of all the volcanic activity?

I suppose a smaller planet would be able to maintain an atmosphere if it has a regular enough supply of icy meteorites to restock it but its hard to see that that could be maintained long enough for complex life to develop.

There are probably only so many classifications that occur in nature so other classifications would need to be for unusual planets that require further investigation. If the demon planet is classified as such because its atmosphere and radiation make it dangerous to both ships and personnel even with protection then those should be considerations taken into account in the classification system.

It's believed that Earth might be toward the low end of the size range for habitable planets, because smaller planets wouldn't have plate tectonics, which is necessary for recirculating carbon from the atmosphere or something like that. An exception might be a large moon of a Jovian in the habitable zone, where tidal stresses might drive plate tectonics in a smaller world than could otherwise have it. But there are questions about whether such large moons would be possible, except in rare cases of initially independent planets being captured into Jovian orbit.

I'm an old timer TOS fan, way before "Minshara" came into the picture. Alls I ever knew was Class M, no explanation given. But way back when I was a kid...

Why M? Only thing I could think of was M being smack in the middle of the alphabet. Class M were the planets just right for human life.

Like some kind of Goldilocks thing, I thought the 26 letter alphabet was the gamut of planets. Class A planets were way too cold, Class Z planets were way too hot, but Class M planets were just right (smack in the middle).

Arbitrary? Probably. But at least I didn't invent some Minshara planets to explain the "M".

Click to expand...

Back in the good ol' days--a.k.a. the 1970's--I assumed (and as I recall, along with other fans, too) that the "M" in "Class M" stood for "Mars", because the Star Trek Writers/Directors Guide said, in so many words, that "Class M" stands for "Earth-Mars conditions".

From the Star Trek Writers/Directors Guide, series created by Gene Roddenberry, third revision, April 17, 1967:

Star Trek Writers/Directors Guide said:

We do not have space suits available or other forms of environmental suits for hostile planet surfaces. These may be obtained for special scripts but keep in mind that we generally restrict our missions to "Class M" planets (approximating Earth conditions).​

[...]

Be creative, but practical here, too. Remember, "Class M" planets will be often similar to many parts of Earth -- and with societies duplicating or intermixing almost any era in man's development. Jungle backgrounds exist on back lots, so what about primeval worlds? Or a pioneer-Indian type culture? Lovely parkland exists locally, so do unusual highly modern buildings, so do farms.​

[...]

I understand the concept of most landings taking place on planets approximating Earth-Mars conditions. But will we never get to a planet where gravity or atmosphere is a problem?

Yes, assuming the right story. Also some story will undoubtedly take us outside our vessel into space for repairs or to investigate some strange object there. But generally we will avoid space helmets and weightlessness since such tales would more legitimately concern Earth's present era of space travel. The aim of our format is drama and entertainment based on character rather than on details of technology and hardware.​